x=5

Let x people have three hobbies.

34+24+48, of which three hobbies are added three times and subtracted twice; If you add two hobbies here, you will subtract 1 times; That is, students with hobbies, plus those without hobbies, are exactly equal to 100.

Solution 2

Let x people have three hobbies.

Y people have 1 hobbies.

x+y+25+( 13+ 14+ 15)/2 = 100

34-x+48-x+24-x = 13+ 14+ 15+y

Get x=5 y=49.

I studied discrete mathematics at that time, and I got high marks in one of the best exams. Now I have forgotten it. There is only one other way, and I hope it can help you.